1. Field of the Invention
This invention relates to the field of measuring and testing, and more particularly to apparatus for the testing of moisture tension in the leaves of growing plants. The invention is directed to the measurement directly of moisture tension in living plants, as opposed to the measurement of total moisture content, or measurement of soil moisture content.
Moisture tension may be defined as the equivalent negative pressure in plant water which is equal to the equivalent pressure that must be applied to the plant water to bring it to hydraulic equilibrium.
2. Description of Prior Art
The measurement of the moisture tension in a growing plant can be a useful indicator of plant stress and other factors that may affect the optimum growing conditions for the plant. It is known that the moisture tension in a plant can vary due to a number of factors, such as the time of day, the amount of photosynthesis taking place within the plant, the ambient relative humidity, and the soil moisture content.
A method for determining moisture tension in plants using air pressure in a sealed system has been described by Dr. Sholander in Science, Vol. 148, pgs. 339-346 (1965). In this system a plant is cut and placed within a pressure vessel. As the air pressure within the vessel increases, the plant sap accumulates on the cut stem. The air pressure at which the plant sap flows is considered the moisture tension and is expressed in atmospheres, sometimes called the negative leaf potential. The vessel in the system described by Dr. Sholander utilizes relatively high pressure and is relatively expensive and dangerous. The procedure is time consuming and also requires that the plant be cut to be placed within the vessel. The instrument of the present invention is intended to measure moisture tension directly in living plants without harming the plant in any way.
A large number of instruments have been described in the patent art for the measurement and testing of non-electrical properties such as moisture. These instruments generally rely upon changes in capacity or resistance due to the presence of moisture. Examples of such patents for the measurement of the moisture content of paper are U.S. Pat. Nos. 2,873,605 and 2,947,166.
Photoelectric instruments have also been used in a wide variety of applications and are known for use in the measurement of moisture content of a vapor stream, for example in U.S. Pat. No. 1,924,139.
There have also been developed a number of instruments for measuring the moisture content of the soil surrounding growing plants, as for example in U.S. Pat. No. 3,951,098. Such devices have also been used in conjunction with automatic irrigation control systems as shown in U.S. Pat. No. 3,910,300.